1. Field of the Invention
The present invention relates to a solution raw material containing at least two organometallic compounds for forming a composite oxide dielectric thin film used for a dielectric memory such as DRAM (Dynamic Random Access Memory), a dielectric filter, and the like, by a metal oxide chemical deposition method (a MOCVD method, a chemical vapor deposition method using an organometallic compound).
2. Description of the Background
As the degree of integration of DRAM increases, use of conventional SiO2 for a dielectric thin film used as a capacitor becomes more difficult, and use of a composite oxide type (an oxide containing at least two metals) dielectric material having a high dielectric constant has been investigated. Examples of such dielectric materials include lead titanate (PT), lead titanate zirconate (PZT), lanthanum lead titanate zirconate (PLZT), strontium titanate (ST), barium titanate (BT), barium strontium titanate (BST), and the like. Particularly, BST is most excellent from the viewpoint of dielectric characteristics.
For thin films of such composite oxide type dielectric materials, a sol-gel method has been conventionally intensively studied in which a film is formed on a substrate by spin coating using metal alkoxides as raw materials. The sol-gel method is capable of easily controlling the film composition because no metal component is vaporized. However, a capacitor electrode of DRAM has a step, and the step increases and is complicated as the degree of integration increases. Therefore, it is difficult to uniformly form a dielectric thin film on the electrode as a substrate by the spin coating method.
Therefore, in recent few years, formation of a dielectric thin film by the MOCVD method with excellent step coverage (the property of adhesion to the surface of a complicated shape having a step) has been actively studied in expectation of higher integration of a device. As an organometallic compound as a raw material, an organometallic complex or a metal alkoxide containing xcex2-diketone compound as a ligand, such as dipivaloylmethane (DPM) or the like, is generally used. Both an alkoxide and xcex2-diketone complex are used as metal raw materials for Ti, Zr, Ta and the like, and xcex2-diketone complexes are mainly used as raw materials for Sr and Ba.
The MOCVD method is a method comprising vaporizing a metal raw material by heating under reduced pressure, transporting the vapor to a deposition chamber, pyrolyzing the vapor on a substrate, and adhering the produced metal oxide to the substrate. In the case of the above composite oxide type thin film, at least two types of organometallic compounds must be used as raw materials. Therefore, control of the amount of the raw material compounds supplied to the deposition chamber, i.e., control of the film composition, is very important for the MOCVD method because compounds have different volatilities.
In formation of the dielectric thin film by the MOCVD method, conventionally, an organometallic compound as a raw material is vaporized by heating, and the generated vapor is transferred to the deposition chamber to form the film. However, an organometallic compound, particularly a DPM complex recommended for use in the MOCVD method, has low stability and volatility, and thus volatility deteriorates during use, or an increase in heating temperature for increasing volatility causes pyrolysis before the vapor of the raw material compound reaches the deposition chamber. Therefore, it is difficult to stably transport the raw material to the deposition chamber, thereby causing a problem in which the expensive raw material is disposed of at each time of film deposition. Also it is difficult to control the film composition, thereby causing a problem in which a thin film having good dielectric characteristics cannot be stably formed.
In order to solve these problems, Japanese Unexamined Patent Publication No. 6-158328 discloses that a solution raw material containing all organometallic compounds dissolved in tetrahydrofuran (THF) is used for the MOCVD method. This solution in a liquid state is supplied to a vaporization chamber arranged before the deposition chamber according to a method referred to as a liquid raw material supply method, and the vapor generated by vaporization in the vaporization chamber is sent to the deposition chamber to form the thin film. In a liquid state, particularly, the DPM complex is stable, and thus the raw material can be repeatedly used. Also, since the heating temperature for vaporization is decreased, it is possible to avoid pyrolysis before the raw material reaches the deposition chamber, and improve controllability of the film composition.
As a similar liquid raw material supply method, Japanese Unexamined Patent Publication No. 5-132776 discloses that a solution of a specified Ba complex (a complex in which two molecules of dipivaloylmethane and one molecule of ethylenediamine are coordinated with a Ba atom) dissolved in THF is used as a raw material. Japanese Unexamined Patent Publication No. 5-247650 discloses that a solution of a metal alkoxide in an organic solvent which boils at the same time as the metal alkoxide is used as a raw material for CVD. Examples of such solvents include alcohols such as ethanol, methanol, propanol, and the like; ethers such as ethyl ether, methyl ether, tetrahydrofuran, and the like; ketones such as methyl ethyl ketone, dimethyl ketone, acetone, and the like. Particularly, ethanol, tetrahydrofuran, and methanol are preferred.
Japanese Unexamined Patent Publication No. 7-321039 filed in 1994 discloses that a flash vapor method (the same as the liquid raw material supply method) comprising dissolving a solid compound as a raw material for CVD in an organic solvent, and sending a small amount of the solution to a vaporizer heated to simultaneously vaporize the solid compound and the organic solvent has been studied for a few years.
Also Japanese Unexamined Patent Publication No. 7503759 discloses that a raw material for CVD is dissolved in a solvent capable of doing co-flash evaporation and is then supplied. Examples of such a solvent include ethers, amines, and alkanols (alcohols). Specifically, isopropanol (IPA), and a solvent mixture of THF/IPA/tetraglyme at 8/2/1 are used.
However, as a result of study conducted by the inventors, it was found that even when a dielectric thin film is formed by the MOCVD method using a solution raw material containing an organometallic compound as a raw material dissolved in THF, composition control is very difficult, and a thin film having the intended composition cannot be easily obtained.
For example, it was found that a raw material such as titanium alkoxide (e.g., titanium tetraisopropoxide) as a Ti raw material, or bis(dipivaloylmethanato) barium complex [Ba(DPM)2] as a Ba raw material tends to react with THF in THF at room temperature to form an nonvolatile reaction product, and the raw material present in the solution is partially vaporized, thereby significantly decreasing the amount of the raw material vaporized.
On the other hand, bis(dipivaloylmethanato) strontium complex [Sr(DPM)2] as a Sr raw material is stable in THF at room temperature. However, since the complex is solvated by the polar solvent in THF, the complex is present in the form of [Sr(DPM)2L1] (wherein L is a solvent, i.e., THF, and n is an integer), and is vaporized in this form. However, in vaporization, the solid raw material [Sr(DPM)2] and the liquid Ln have greatly different evaporation temperatures, and thus Ln is dissociated by heat in the course of evaporation to easily cause the phenomenon that the Sr raw material is not sent to the deposition chamber. Also THF is polymerizable, and thus produces ring opening polymerization by heating in evaporation, thereby making the complex unstable.
Therefore, the use of the THF solution makes unstable the supply of a raw material vapor to the deposition chamber, and has the large possibility of producing a composite oxide having a metal composition ratio completely different from the composition ratio of the metal raw materials in the solution. It is thus difficult to control the film composition, and it is still impossible to realize stable production of a dielectric thin film having a predetermined composition. The use of the above solvent mixture of THF/IPA/tetraglyme at 8/2/1 containing THF as a main component produces substantially the same results. A solution raw material containing alkanol such as IPA as a solvent also has difficulties in controlling the composition.
It is an object of the present invention to solve this problem. More specifically, it is an object of the present invention to provide a solution useful for producing a dielectric thin film by the MOCVD method having excellent controllability of film composition and permitting stable supply of the above organometallic compounds as raw materials to the deposition chamber without producing pyrolysis of the organometallic compounds as raw materials and nonvolatile products, thereby permitting the secure formation of a dielectric thin film having a predetermined composition.
The present inventors found that by using a polar solvent such as a cyclic or acyclic diether, alkyl-substituted cyclic monoether, mono- or di-substituted alkyl monoether, alkoxy alcohol, diol, or acetoacetic ester, or a solvent mixture of a non-polar solvent such as saturated hydrocarbon and one of these polar solvents in place of THF, making the Ti raw material and Ba raw material nonvolatile, and dissociation of the Sr raw material are eliminated or significantly decreased, thereby permitting stable supply of vapors of these raw materials to the deposition chamber. This resulted in achievement of the present invention.
The present invention therefore provides a solution raw material for forming a composite oxide type dielectric thin film by the MOCVD method, comprising a solution containing at least one organometallic compound as a raw material dissolved in at least one solvent selected from cyclic or acyclic diethers, alkyl-substituted monoethers, mono- or di-branched alkyl monoethers, alkoxy alcohols, diols, and acetoacetic esters.
In another embodiment, the present invention provides a solution raw material for forming a composite oxide type dielectric thin film by the MOCVD method, comprising a solution containing at least one organometallic compound as a raw material dissolved in a solvent mixture containing at least one solvent selected from cyclic and acyclic saturated hydrocarbons, and at least one solvent selected from cyclic or acyclic diethers, alkyl-substituted monoethers, mono- or di-branched alkyl monoethers, alkoxy alcohols, diols, acetoacetic esters, and unsubstituted or alkyl-substituted pyridine.